1. Field of the Invention
The present invention relates to a waveform equalizing filter unit for use in equalizing waveform distortions or the like in a decoder employed in a high definition television (HDTV) system and, more particularly, to a waveform equalizing filter unit capable of selectively switching clock signals in accordance with the distortion characteristic.
2. Description of the Prior Art
It is known that waveform equalizing filters are employed in a variety of signal processing fields inclusive of a communication system. Out of such waveform equalizing filters, the one most commonly used is a type termed a transversal filter.
First a description will be given with regard to an exemplary case of applying such waveform equalizing filter to a MUSE (Multiple Sub-Nyquist Sampling Encoding) decoder in a receiver of the HDTV system. The MUSE format is based on a sample value transmission method which rearranges samples of an input video signal and transmits the same in the form of analog sample values. (Refer to Nikkei Electronics, No. 433, Nov. 2, 1987, p. 192) Transmission distortion causes interference between sample values and induces ringing on a reproduced image. Effective removal of such ringing is achieved by the provision of a waveform equalizing filter in a MUSE decoder to perform equalization of the waveform. An exemplary circuit configuration of a MUSE decoder is shown in FIG. 6. (Refer to Iwadate et al., "Built-in Type Waveform Equalizer for MUSE Decoder", 1988 National Convention of Television Society, 16-6, pp. 351-352)
The MUSE decoder of FIG. 6 comprises a low-pass filter 11, a clamping circuit 12, a main-line signal A-D (analog-to-digital) converter 13, a delay circuit 14, an FET (Field effect transistor) amplifier 16, a sub-line signal A-D converter 17, a waveform equalizing filter 18, a switch 19, an adder 15, a VIT (vertical interval test) signal extractor 20 and a microcomputer 21 which are connected as shown. Since the MUSE transmission rate is 16.2 MHz, sampling by the main-line A-D converter 13 is executed at a frequency (rate) of 16.2 MHz. Meanwhile, waveform equalization needs to be performed at a sampling frequency of, in principle, at least 32.4 MHz which is double the MUSE transmission rate. Therefore the sampling frequency of the sub-line A-D converter 17 is set to 32.4 MHz. Consequently the sampling frequency of the waveform equalizing filter 18 is set also to 32.4 MHz.
The switch 19 serves to apply an output signal of the waveform equalizing filter 18 at a VIT timing to a main-line signal, and in accordance with the VIT signal extracted by the VIT signal extractor 20, the microcomputer (MC) 21 determines a tap coefficient of the waveform equalizing filter in response to a reference impulse multiplexed in the vertical blanking period of the MUSE signal, thereby setting the tap coefficient in a coefficient multiplier of the waveform equalizing filter so as to cancel the transmission distortion.
The distortion to be equalized varies widely depending on the signal transmission line and the signal distribution line. For example, in satellite broadcasting where distortion is small, the sampling frequency of the sub-line A-D converter and that of the waveform equalizing filter are both 32.4 MHz to realize fine waveform equalization. Meanwhile, in case a CATV network is utilized, great deterioration is caused in a reproduced image due to reflection in the distribution line or distortion in the transmission line, hence necessitating waveform equalization in a temporally long range. In such a case, a sampling frequency of 16.2 MHz may be sufficient. (Refer to Ito et al., "Waveform Equalizer for MUSE Signal Transmission", Technical Report in Television Society, Oct. 26, 1989, pp. 13-17)
Thus, the waveform equalizing filter in the MUSE decoder needs to have an adequate equalization characteristic which conforms with the distortion characteristic. Consequently it is necessary to incorporate a plurality of waveform equalizing filters in the MUSE decoder and to use them selectively. However, since each waveform equalizing filter is composed of an expensive transversal filter with multiple taps to perform a fast operation, it is difficult to provide a plurality of such filters. Considering in particular that a MUSE decoder is incorporated in a television receiver of a general user, the provision of such filter brings about a rise in the cost of each television receiver. It is therefore not practical to employ a plurality of waveform equalizing filters in each television receiver.
The problem mentioned above is not limited to a MUSE decoder alone, but also arises in regard to any other communication system.